Since polyvinyl alcohol (to be referred to as PVA hereinafter)-based resins are excellent in gas barrier property, toughness, transparency and the like, they are suitable as packaging material of various articles.
However, since a general PVA-based resin has the melting point close to the degradation temperature, its melt molding is substantially impossible, and as its molding method, a method in which it is made into an aqueous solution, and then this is cast and dried to make a film or coated on the surface of various substrate, followed by drying them have to be employed, and this limitation has been a great obstacle in broadly developing the PVA-based resins into packaging material applications.
Contrary to this, a PVA-based resin comprising a 1,2-diol component in its side chain has been proposed in recent years as a PVA-based resin which can be melt molded and is excellent in gas barrier property (e.g., see Patent Literature 1).
The high melting temperature and excellent gas barrier property of PVA-based resins arise from high crystallinity due to simple molecular structure and strong constraint of the molecular chain by the hydrogen bond between hydroxyl groups in the crystalline moiety and amorphous moiety. It is considered that such a PVA-based resin described in the Patent Literature 1 has a reduced melting point due to reduced crystallinity caused by the steric hindrance of the side chains, but in spite of this, the reduction of gas barrier property due to reduction of crystallinity is inhibited by the strong hydrogen bond by the hydroxyl groups in the side chains in the amorphous moiety.